Amplifier circuits



Feb. 9 1926.

' H. S. READ AMPLIFIER CIRCUITS Original Filed April 27, 1920 .Patented Feb. 9, 1926.

UNITED STATES PATENT OFFICE.

HARRY S. READ, OF ITHACA, NEW YORK, ASSIGNOR TO WESTERN ELECTRIC COM- PANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

AMPLIFIER CIRCUITS.

Application ledApril 27, 1920, Serial No. 376,909. Renewed April 8, 1925.

To cZZ 'whom it may con-cern.:l

Be it known that I, HARRY-S. READ, a citizen of the United States, residing at Ithaca, in the county of Tompkins and State of New York, have invented certain new and useful Improvements in Amplifier Circuits, of which the following is a full, clear, concise, and exact description.

The invention relates to amplifying systems and has for an object to provide a systeni capable of amplifying the voice energy from one loud speaking transmitterto aF magnitude sufficient to operate a large number, for instance, as many as 100 loud speaking receivers.

The invention makes use of vacuum tube amplifiers and provides an amplifier set that is capable of giving a very large output. In the use of the audion type amplifier, it has been common practice heretofore to maintain the grid or control electrode at all times at a negative potential with respect to the cathode, in order that none of the electrons emitted by the cathode will go to the grid. This prevents a flow of current in the gridcathode circuit, thereby providing a high input circuit which consumes a very small amount of energy. l .i A

According to prior practice, where the grid is always maintained negative,` it has been possible to work the vacuum tube arnplifiers only over a very limited range, for the incoming potentials have always been maintained small enough to prevent overcoming the normal steady negative potentialv of the grid. vThis limitation is not imposed on the circuit provided according to the present invention, however, which embodies a circuit working the amplifier over a very wide range of voltages, thereby deriving from the vacuum tube amplifier an appieciably greater amount of energy.

According to this invention, the grid is not maintained normally at 'a negative potential with respect to the cathode, but the grid is instead at the saine potential as the negative end of the cathode in the absence of electrical impulses to be repeated. In this case an electrical` impulse-appliedto tlie cathode grid circuit will, at times, give fthe grid a potential which is positive Witlirespeet to the cathode. At such times as the `grid is positive, current flows in the cathodegrid circuit` v'and the impedance of the cathode-grid circuit becomes much lower than at a time when the grid is negative. It is therefore evident that more energy mustbe supplied to the grid-cathode circuit to establish a given positive potential on the grid than an equal negative potential, and that the amount of this energy will vary with the magnitude of the potential to be established. From a slightly different viewpoint this means that the instantaneous impedance of the grid-cathode circuit varies from point to point along the applied positive voltage wave, such variations in practical amplifiers for voice frequencies bein niegohms to a few thousan ohms. It will now be clear that if the input or supply circuit have sucli a high impedance that the internal voltage drop due to drawing current to the grid is appreciable, a very serious distortion of the positive half wave will result. Distortion from this source approaches zero as this supply circuit is made to approach zero impedance. For high grade tone quality and articulation of re cated voice currents it has been found su cient, in amplifiers of this type, to make the iinpedance of the connected line or supply circuit of the same order of impedance as' the minimum impedance reached by the gridcathode circuit during the positive halt` wave.

It is of course realized that with the lower impedance of the input circuit more energy willbe needed to supply a given potential to the grid circuit. This can readily be compensated for by employing one or two tubes to supply the energy to the grids ofniany tubes in multiple each operating at or near its maximum energy output. yIt has been found that instead of adding more tubes in parallel in af nhigh power amplifier having the tubes operated with the grids negative, it

is by'far more efficient to add this auxiliar)7 amplifying stage for driving the grids positive.

For further details of the invention, reference vmay be made to the drawings in which Fig. 1 represents diagrammatically a. vacuum tube amplifier system embodying the features of this invention. Fig.'2 is a curve showing the relation between the grid voltage and the plate current of a vacuum tube amplitier. t

from several,

Referring in detail to Fig. 1, tliecircuit 1 i tively weak impulses in the line 1 sufficiently to operate a large number of receivers. The line 1 is connected by transformer 9 to two vacuum tubes 10 and 11 in parallel.

The resistance 12 for speech amplifier design may have a value of the order of 12,000 ohms, this being such that the minimum impedance of the grid-cathode circuit will be about equal to the absolute value of the imedance of this resistance and the line transormer in parallel. The vacuum tubes 10 and 11 are coupled by means of transformer 13 to a large number, for instance, ten vacuum tubes 14, 15, etc., connected in multiple and working into the utput transformers 16, 17,718, 19 and 20.l The secondary winding of transformer 13, shown with two secondary windings .in parallel, has the propere turns ratio to transform the impedance of the first stage to match the minimum 1mpedance of the the grid circuit of the multiple audions 14,15, etc. The value of th1s minimum impedance will depend .upon the number and the type of tubes employed, as well as the value of the impressed potential, but in the apparatus illustrated, it ma be said that the impedance of these multlpled tubes will be of the order of two or three thousand ohms. It may be desirable in some eases to` shunt the grids 14, 15, etc., by av resistance .corresponding to 12 and its value would be determined by the same criterion. If it is necessary to 'employ the `amplifier for purposes which admitof less distortion than the reproduction of speech for intelligible audition, the values of these resistances may of course be' reduced and the transformer ratios modified accordingly.

Referring to Fig. 2, this shows the relation between the grid voltage (EC) and the plate current (IB milliamperes) for each of `the .tubes which havebeen employed inthe ,The curve shows that the tubes could have av circuit of Fig. 1 with good results.

range of $30 volts peak value, about the value -`30"volts, in the grid circuit without causing the gridto become positive with respect to the filament. The corresponding plate current would be about 40 milliamperes per tube peak value. This would correspond to about 1.15 watts peak value perv tube. By allowing the grid to become positive, the voltage range 1s increased lto i60 volts grid voltage, about the value 0 volts,

and the plate current is increased to approxipower is -increased to 7.1 watts peak value per tube. It is apparent that by operating the grid with positive potentials the power output 1nl this case is multipled practically by 6. The potential supplied by circuit 1,

Fig. 1, should be such asto drive the grid-to a large positive potential in the case of the tube having the "charactertistic illustrated in- Fig. 2. This potential may be of the order of 60 volts. j U' Considerable advanta e is to be derived from connecting (as in. ig.1) two 4vakcuum tube sets in tandem, each set comprising a plurality of tubes in parallel.. In the above mentioned instance, where two vacuum tubes in parallel were connected in tandem to ten vacuum tubes in parallel, it was found possible to operate the one hundred and three 2500-ohm loud speaker'receivers eventhough one amplifier in the first set and four amplifiers inthe second set were removed. Thus it would be possible during operation' of the circuit for one amplifier in the first set and four in the second set tobecome inoperative due 'to burning out the filament, breaking p the glass envelope, etc., without rendering the circuit' as a whole inoperative. This is pllcation, claims broadl the feature of having the impedance w ich faces the gridcathode internal impedance of a vacuuml tube low, in order that the impe-dance across the grid and cathode may not vary unduly when the grid becomes positive. lfVhat is claimed is: 1. vAn incoming line adapted to supply.'

erial No. 432,444, filed December 22,1920, assigned to the asslgnee of this ap- 100 l impulses to be amplified, a vacuum tube amplifier havin control electro e, means connecting said a cathode, an anode :md la.

control electrode and said cathode to said said imnected to' said control electrode and said cathode bemg approximately the same as the impedance o the cuit when `the control electrode is at its maximum positive potential.

A 2. An incoming line adapted pulses to'lbe amplified, a vacuum tu e amcontrol `electrmie cir'- to sup ly illu.

plifier comprising a cathode, an anode andv a control electrode, and means connecting iso said cathode and said control electrode to said line and maintaining the steady voltage between said cathode and said control electrode at such a value that said amplifier operates on only the substantially straigl'it part of its input voltage-anode current characteristic in repeating said impulses, thel magnitude of the impulses supplied by said line being sufficient to make said grid positive at times, lthe control electric circuit exterior to said vacuum tube having an impedance of the order of the impedance between said control electrode and cathode when said con'- trol electrode is at a positive potential.

3. In combination, a vacuum tube amplifier having a cathode, an anode and a control electrode, an incoming line connected to said electrode for supplying alternating current impulses to said electrode, and means maintaining said cathode and said electrode atsubstanti-ally the same potential in the absence of said impulses, and causing said ainplifier to operate on only the/substantially straight part of its input voltage-anode cui'- rent characteristic in repeating said' im,- pulses, the impedance of said line connected to said yelectrode being approximately the 'same as the minimum impedance of said amplifier bet-Ween said electrode and said cathode.

4. An incoming line adapted to supply impulses to be amplified, avacuum tube amplifier, means inductively connecting. said amplifier to said line, said amplifier having a control electrode, the magnitude of the impulses of said line. being sufficient to make the potential of said control electrode at times positive,` and means on the input' side cf' said amplifier for reducing the distortion otthe Wave When said grid is positive With respect to said cathode and causing said amplifier to operate on only thesubstantially straight part of its input voltage,-l

anode current characteristic in repeating said impulses.

5. The method which comprises assigning to the grid of a vacuum tube amplifier a zero steady potential during normal operation of the amplifier, energizing said amplifier With alternating current impulses, compensating` forthe loss occasioned bythe consumption of energy in the grid' circuit when said grid is at a positive potential with respect to its cathode, and approximately inatchinotlie impedance of the incoming line to fhatyof the minimum impedance of' said `grid circuit.

6. The method which comprises supplying the grid circuit of a vacuum tube amplifier with alternating current impulses of value sufiiciently to make the potential of said grid positive With'respect to its cathode, approximately matching the impedance of the incoming line to that of an impedance value of said grid circuit when said grid is posi- Vsaid grid circuit and maintaining the steady potential of said grid at sucha value that thel amplifier operates on only the substantially straight 'part of its grid potentialanode current characteristic in repeating said impulses.

8. The. method Which comprises supplying the grid circuit of a vacuum tube amplifier with alternating current impulses of value sufficient to make the potential of said grid positive with respect to itscathode, approximately matching the impedance of the incoming line to that assumed by said grid circuit when said grid is at a positive potential, and compensating for the loss occasioned by ythe consumption of energy in said grid cir- 'cuit due to the potential of said grid becoming positive with respect to its cathode. and maintaining the steady potential of said grid at such a value that the ampli-fier operl ates on only the substantially straight part' circuit is at a positive potential, and said coupling comprising means maintaining the steadyy potential of said grid at such a value that the amplifier operateson only the substantially straight part of its grid potential-anode current characteristic in repeating said impulses. J

10. The combination of a'lin'e for' supplying electricalvariations'tfa vacuum tube amplifier connected thereto having means for maintaining its `ste ity inputvoltage at such a value 'that said amplifier operates' on' only the substantially straight part of its'input voltage-anode current chai'- acteristic in amplifying said variations. a' second vacuum tube amplifier, for further amplifying said variations, said second amplifier having means for maintaining its steady input voltage at such a value that the amplifier operates on only the substantially straight part of .its inputl voltagetions that the grids of said circuits are positive.

,1L The combination of a source of current lmpulses, a Vacuum tube ampliher connecty ed thereto, a substantially ure 'electron discharge vacuum tube ampli er having a cath- 0de and1a grid, and a coupling between ,said amplifiers, said coupling having an impedance `substantially matching the minimum grid-cathode impedance of said second mentioned'ampliier, and said coupling compris- .ing means maintaining zero normal steady potential on the grid of said second amplifier and causing that amplifier to operate on only the substantially straight part of itrs grid "potential-anode current characteristic in repeating said impulses.

12. The combination of a line, a vacuum tube ampliier stage connected thereto, a second vacuum tube amplifier 'stage 4for repeating without substantial change of wave form, electrical variations delivered thereto from said first mentioned amplifier stage, and a transformer between said s. ges, said second stage having its input elec rodes, in-

clding a control electrode, at the `samel steady potential during normal operation of the systems, and the impedance at the sec-l ondary circuit of said transformer substantially matching the impedance facing said circuit from said electrodes under the condition that said control electrode vis posi- HARRY S. READ. 

